1. Field of the Invention
The present invention relates to a process for depositing polycrystalline silicon.
2. Background Art
When polycrystalline silicon is prepared by chemical gas phase deposition of chlorinated silanes, for example trichlorosilane or dichlorosilane, by decomposing the gases over glowing silicon surfaces such as rods (Siemens process) or granules (fluidized bed process), silicon deposits on the hot surfaces in the primary reaction, and tetrachlorosilane forms as a by-product. The deposition of polycrystalline silicon from trichlorosilane is based on thermal equilibrium processes of chlorosilanes. For example, in trichlorosilane deposition, depending upon the reaction conditions, the main reactions are:4 SiHCl3 - - - >3 SiCl4+Si+2 H2 or4 SiHCl3+2 H2 - - - >3 Si+SiCl4+8 HCl.
Chlorosilanes which can be condensed as a liquid from the offgas (“offgas condensate”) of the silicon deposition reactors include, in addition to dichlorosilane, trichlorosilane and silicon tetrachloride, depending upon the deposition conditions, 0.001-3% by weight of high-boiling chlorosilanes, (“high boilers”), which are formed in side reactions. High-boiling chlorosilanes are compounds which consist of silicon and chlorine, with or without hydrogen, oxygen and carbon, and have a higher boiling point than tetrachlorosilane (57° C. at 1013 hPa). They are preferably disilanes of the formula HnCl6-nSi2 (n=0-6), oligo(chloro)silanes of the formula H2n-mClmSin (n=2 to 68, preferably 2 to 4 and m=0 to 2n), disiloxanes of the formula HnCl6-nSi2O (n=0-4), siloxanes of the formula H3Si—[O—SiR2]n—O—SiH3 (n=1 to 4, preferably 1 or 2; R is independently H, Cl or CH3), and cyclic oligosiloxanes of the formula
and the methyl derivatives thereof. In a typical composition, these high-boiling chlorosilanes consist of about 50% by weight of Si2Cl6, >34% by weight of Si2HCl5, 10% by weight of Si2H2Cl4 (2 isomers), 5% by weight of Si2H2Cl3 (2 isomers) and <1% by weight of even higher-boiling chlorosilane components.
For the processing of the offgases from polysilicon deposition, various processes are known: DE2918066 describes a process in which all chlorosilanes obtained in the condensate from the polysilicon deposition are supplied back to the reactant gas of the deposition. The serious disadvantage of this process is the extremely low deposition rate of silicon, which is caused by the high concentration of tetrachlorosilane which arises in the equilibrium in the reactant gas, which makes the deposition process uneconomical (W. C. O'Mara, R. B. Herring, L. P. Hunt, Handbook of Semiconductor Silicon Technology, ISBN 0-8155-1237-6, p. 77, 1990).
In the commercially used process known as the Siemens process for preparing rod-shaped polycrystalline silicon by means of trichlorosilane deposition, it is therefore customary to only recycle, chlorosilanes with a boiling point lower than the boiling point of trichlorosilane, together with the unreacted trichlorosilane from the offgas, back to the Siemens deposition reactor for preparation of polycrystalline silicon. The tetrachlorosilane obtained is removed by distillation from the offgas stream and either converted to trichlorosilane (Motorola, U.S. Pat. No. 3,933,985) or used as a starting material for other chemical products, for example fumed silica or tetraethyl silicate (cf. Handbook of Semiconductor Silicon Technology, ISBN 0-8155-1237-6, p. 72, 1990). The high boilers which are likewise obtained are either disposed of (e.g. U.S. Pat. No. 4,252,780) or converted to monomers. This is done either by reaction with tetrachlorosilane and hydrogen, or by cracking with HCl (Osaka Titanium, JP Hei 1-188414; Tokuyama, JP H09-263405; Union Carbide, U.S. Pat. No. 4,340,574; Hemlock, WO 02/100776 A1).
It is also known that high-purity hexachlorodisilane (H2Si2Cl6) can be isolated from the offgases of the deposition of polycrystalline silicon (WO 2002012122). However, the isolation of these high boiler fractions as a starting material for specific epitaxy applications or for preparation of silicon polymers is very complicated.
All processes of this type for processing high boilers are associated with yield losses, especially of chlorine and silicon, environmental pollution by hydrolysis products, or complicated plants and processes. Moreover, in the recycling processes described to give monomers, the semiconductor purity of the compounds present in the condensate is lost. This first has to be reestablished by means of complicated purification steps, preferably by distillation, before the products can be used again in the deposition process.
One economically viable use of high boilers is described by DE 102006009953, where the high boilers are used to prepare fumed silica. A disadvantage is that along with the preparation of polysilicon, the preparation of fumed silica also has to be conducted, which requires a “coupling” of these different products, which is not always desired.